1. Field of the Invention
The present invention relates to variable displacement compressors which are employed in motor vehicle air-conditioning systems, for instance.
2. Description of the Related Art
FIG. 10 shows a conventionally known structure employed in a variable displacement compressor of this kind, in which cylinder bores 101a are formed in a housing 101, a drive shaft 102 is rotatably supported in the housing 101, a rotary support 103 is fixed to the drive shaft 102, a cam plate 104 is supported by the drive shaft 102 which is passed through a through hole 104a formed in the cam plate 104, and pistons 105 fitted in the individual cylinder bores 101a are joined to the cam plate 104. A maximum inclination setting projection 106 protrudes from about a point Db of the cam plate 104 corresponding to a bottom dead center toward the rotary support 103.
A hinge mechanism 107 comprises guide pins 108 provided close to a point Da of the cam plate 104 corresponding to a top dead center and supporting arms 109 provided on the rotary support 103 corresponding to the guide pins 108. The guide pins 108 are firmly press-fitted into holes formed in the cam plate 104 and have bulbous parts 108a at extreme ends. On the other hand, guide holes 109a are formed in the individual supporting arms 109. The guide pins 108 and the supporting arms 109 are joined together as the bulbous parts 108a of the former are fitted into the respective guide holes 109a of the latter.
With the bulbous parts 108a of the guide pins 108 fitted into the guide holes 109a in the individual supporting arms 109, the cam plate 104 can rotate together with the drive shaft 102. Thus, rotary motion of the drive shaft 102 is converted into reciprocating motion of the pistons 105 in the cylinder bores 101a by way of the rotary supports 103, the hinge mechanism 107 and the cam plate 104. As a consequence, a refrigerant gas is introduced into the cylinder bores 101a, compressed, and discharged in repeated cycles.
The drive shaft 102 supports the cam plate 104 in such a way that the cam plate 104 can vary its angle of inclination while sliding along the drive shaft 102. This is because the bulbous parts 108a of the individual guide pins 108 and the guide holes 109a of the hinge mechanism 107 work as slide guides and the through hole 104a allows the cam plate 104 to slide along the drive shaft 102. The stroke of the pistons 105 and, thus, the displacement capacity of the compressor are varied by adjusting the angle of inclination of the cam plate 104. When the maximum inclination setting projection 106 of the cam plate 104 comes into contact with the rotary support 103, the cam plate 104 is restrained from sliding and inclining further, where the cam plate 104 reaches its maximum angle of inclination.
When the cam plate 104 is set to its maximum angle of inclination, the stroke of the pistons 105 increases so that the refrigerant gas compression ratio also increases. As a result, a large compressive load acts on the supporting arms 109 by way of the pistons 105, the cam plate 104 and the guide pins 108 so that the guide pins 108 receive a high level of reaction force from the supporting arms 109 which sustain the compressive load. The guide pins 108 employed in the conventional structure have a large diameter, for instance, so that they should be able to withstand the large reaction force. Furthermore, portions of the cam plate 104 where the guide pins 108 are fitted are made thicker to provide a sufficient mechanical strength for supporting the guide pins 108.
The use of the large-diameter guide pins 108, which have naturally a heavy weight, combined with the thickening of the portions around the holes in which the guide pins 108 are fitted results in a considerable increase in the weight of the cam plate 104. In addition, there is the need to fit a large counterweight to make up for an unbalanced weight distribution around an axis L of the drive shaft 102 caused by the provision of the guide pins 108 and the thickened portions of the cam plate 104. This also causes an increase in the weight of the cam plate 104. A major problem resulting from such increase in the weight of the cam plate 104 in the conventional structure has been the delay in altering the angle of inclination of the cam plate 104, or deterioration of the controllability of the displacement capacity of the compressor.
According to a proposal made in recent years, the weight of the compressor can be reduced by forming the cam plate 104 with an aluminum-based metallic material. The cam plate 104 formed of the aluminum-based metallic material has a lower stiffness than conventionally used iron-based metallic materials, however. It has therefore been difficult to produce the guide pins 108 with a desired length of fit which would be required for securely press-fitting the guide pins 108 into the cam plate 104 and, as a consequence, the guide pins 108 have tended to be mounted with low strength. Accordingly, what is important for producing the cam plate 104 with the aluminum-based metallic material is to lower the ratio of a load supported by the hinge mechanism 107 to a maximum compressive load acting on the cam plate 104 when the compressor is operated at its maximum displacement capacity.